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SSL_EXPORT_KEYING_MATERIAL(3)       OpenSSL      SSL_EXPORT_KEYING_MATERIAL(3)
NAME
       SSL_export_keying_material, SSL_export_keying_material_early - obtain
       keying material for application use
SYNOPSIS
        #include <openssl/ssl.h>
        int SSL_export_keying_material(SSL *s, unsigned char *out, size_t olen,
                                       const char *label, size_t llen,
                                       const unsigned char *context,
                                       size_t contextlen, int use_context);
        int SSL_export_keying_material_early(SSL *s, unsigned char *out, size_t olen,
                                             const char *label, size_t llen,
                                             const unsigned char *context,
                                             size_t contextlen);
DESCRIPTION
       During the creation of a TLS or DTLS connection shared keying material
       is established between the two endpoints. The functions
       SSL_export_keying_material() and SSL_export_keying_material_early()
       enable an application to use some of this keying material for its own
       purposes in accordance with RFC5705 (for TLSv1.2 and below) or RFC8446
       (for TLSv1.3).
       SSL_export_keying_material() derives keying material using the
       exporter_master_secret established in the handshake.
       SSL_export_keying_material_early() is only usable with TLSv1.3, and
       derives keying material using the early_exporter_master_secret (as
       defined in the TLS 1.3 RFC). For the client, the
       early_exporter_master_secret is only available when the client attempts
       to send 0-RTT data. For the server, it is only available when the
       server accepts 0-RTT data.
       An application may need to securely establish the context within which
       this keying material will be used. For example this may include
       identifiers for the application session, application algorithms or
       parameters, or the lifetime of the context. The context value is left
       to the application but must be the same on both sides of the
       communication.
       For a given SSL connection s, olen bytes of data will be written to
       out. The application specific context should be supplied in the
       location pointed to by context and should be contextlen bytes long.
       Provision of a context is optional. If the context should be omitted
       entirely then use_context should be set to 0. Otherwise it should be
       any other value. If use_context is 0 then the values of context and
       contextlen are ignored.  Note that in TLSv1.2 and below a zero length
       context is treated differently from no context at all, and will result
       in different keying material being returned.  In TLSv1.3 a zero length
       context is that same as no context at all and will result in the same
       keying material being returned.
       An application specific label should be provided in the location
       pointed to by label and should be llen bytes long. Typically this will
       be a value from the IANA Exporter Label Registry
       (<https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels>;).
       Alternatively labels beginning with "EXPERIMENTAL" are permitted by the
       standard to be used without registration. TLSv1.3 imposes a maximum
       label length of 249 bytes.
       Note that this function is only defined for TLSv1.0 and above, and
       DTLSv1.0 and above. Attempting to use it in SSLv3 will result in an
       error.
RETURN VALUES
       SSL_export_keying_material() returns 0 or -1 on failure or 1 on
       success.
       SSL_export_keying_material_early() returns 0 on failure or 1 on
       success.
HISTORY
       The SSL_export_keying_material_early() function was added in OpenSSL
       1.1.1.
COPYRIGHT
       Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at
       <https://www.openssl.org/source/license.html>;.
1.1.1k                            2021-03-25     SSL_EXPORT_KEYING_MATERIAL(3)